Terbium luminescence studies of binding of adriamycin and cisplatin to tumorigenic cells

The luminescent properties of terbium ions are used to investigate the interaction of adriamycin and cisplatin with GH3/B6 pituitary tumor cells. Clinically relevant concentrations of adriamycin were found to quench the intensity (IC₅₀ = 0.6 μM) and excited-state lifetime (τ/τ₀ = 0.73) of the Tb³⁺—GH3/B6 complex. Inspection of the Tb³⁺—GH3/B6 emission spectrum and the visible absorption spectrum of adriamycin strongly strongly suggests that the quenching of Tb³⁺ luminescence by adriamycin is due to dipole-dipole resonant energy transfer; and, according to Forster's theory (R₀ = 33.6 Å), the adriamycin receptor site is located ca. 40 Å away from the bound probe, at the lipid/protein interface. The quenching of Tb³⁺ luminescence by cisplatin is best explained by a static energy-exchange mechanism; in that the cisplatin receptor site is contiguous with the Tb³⁺ binding site at the outer surface of the membrane. The data suggest that, in the plasma membrane of tumorigenic cells, the adriamycin and ciplatin receptor sites are intimately associated with the same calcium-binding protein.